1. Field of the Invention
The invention relates to a method for forming a thin film transistor (TFT) and, more particularly, to a method for forming a thin film transistor with a lightly doped drain (LDD) structure.
2. Description of the Related Art
At present, methods for forming liquid crystal display (LCD) devices and panels are mainly based on polysilicon thin film transistor technologies.
FIGS. 1A through 1F sequentially show steps for forming a thin film transistor for an LCD device. As shown in FIG. 1A, a polysilicon layer 102 is formed on a glass substrate 100. A gate insulating layer 104 and a gate electrode 106 are sequentially formed on the polysilicon layer. As an example, the gate insulating layer 104 and the gate electrode 106 can be formed by sequentially depositing nitride and polysilicon on the polysilicon layer 102, and then patterning the deposited nitride and polysilicon layer.
Next, as shown in FIG. 1B, N-type impurities 108 are implanted to the polysilicon layer 102, to form a first ion-implant region 110, as shown in FIG. 1C. After that, an oxide layer (not shown) is deposited on the gate insulating layer 104 and the gate electrode 106, and the oxide layer is then patterned to form sidewall spacers 112 on both sides of the gate electrode 106, as shown in FIG. 1D.
Subsequently, as shown in FIG. 1E, using the sidewall spacers 112 as masks, N-type impurities 114 are implanted in the polysilicon layer 102, to form a second ion-implant region 116, as shown in FIG. 1F. The doping concentration of the second ion-implant region 116 is higher than that of the first ion-implant region 110. The first ion-implant region 110 with lower doping concentration is called a lightly doped drain region.
One object of the invention is to provide a method for forming a thin film transistor with a lightly doped drain structure; the manufacturing steps are simplified than in prior art methods. Therefore, the yield of the thin film transistor can be increased and the production costs can also be lowered.
In accordance with one example of the invention, the method for forming a thin film transistor comprises the steps of: forming a gate insulating layer and a gate electrode on a polysilicon layer; forming a photoresist layer with a predetermined thickness on the gate electrode and on a portion of the polysilicon layer; and implanting first conductive type impurities into the polysilicon layer so as to form a first ion-implant region and a second ion-implant region, wherein the doping concentration of the second ion-implant region is higher than that of the first ion-implant region.
In accordance with another example of the invention, the method for forming a thin film transistor comprises the steps of: forming a first polysilicon layer and a second polysilicon layer on a glass substrate, respectively corresponding to a predetermined region for first conductive type thin film transistor and a predetermined region for second conductive type thin film transistor; forming a first gate insulating layer and a first gate electrode on the first polysilicon layer; forming a second gate insulating layer and a second gate electrode on the second polysilicon layer; forming a first photoresist layer with a first predetermined thickness and a second photoresist layer with a second predetermined thickness with the use of a gray level mask, wherein the first photoresist layer is formed on the first gate electrode and on the first polysilicon layer, the second photoresist layer is formed on the second gate electrode and on a portion of the second polysilicon layer, and the first predetermined thickness is larger than the second predetermined thickness; and implanting first conductive type impurities the second polysilicon layer so as to form a first ion-implant region and a second ion-implant region in the second polysilicon layer, wherein the doping concentration of the second ion-implant region is higher than that of the first ion-implant region.